8418钢铝合金压铸模早期开裂失效分析

Failure analysis of early cracking of aluminum alloy casting die made of 8418 steel

采用直读光谱仪、数显洛氏硬度计、光学显微镜、扫描电镜及能谱仪,对8418钢铝合金压铸模具失效件的化学成分、表面缺陷形貌特征及显微组织进行检测和分析。结果表明,经过理化检测,基体的晶粒组织粗大,粗大晶粒的平均直径达1647μm,材料过热程度异常严重。原始粗大晶粒带来最终热处理淬火应力的增大,因而产生解理特征的裂纹以及沿晶韧窝的断口形貌。在后期缓冷过程中,碳化物沿晶析出,使得晶间强度显著降低,在淬火组织应力的影响下,形成沿晶开裂的内裂纹。铝合金压铸模早期开裂的原因,是由于过热粗大组织及沿晶开裂的内裂纹,大幅度降低材料强度韧性。模具在使用过程中难以承受压铸加工过程的工作应力,最终在模具模腔表面圆角最小处,即应力集中倾向最大的部位产生开裂。

The chemical composition,surface defect morphology and microstructure of failure parts of the aluminum alloy die-casting mold made of 8418 steel were analyzed by means of direct reading spectrometer,digital Rockwell hardness tester,optical microscope,scanning electron microscope and energy spectrometer.The results show that,the grain size of matrix is coarse.The average diameter of coarse grains is 1647μm,and the degree of material superheat is extremely serious.The original coarse grains lead to an increase of the quenching stress after the final heat treatment,and thus the fracture morphology of cleavage characteristic and intergranular dimple are observed.Carbides were precipitated along grain boundaries during the subsequent slow cooling,as a result,the intergranular strength is decreased remarkably.Under the influence of the quenching microstructure stress,internal crack along the grain boundary is formed.The reason for the early cracking of the aluminum alloy die-casting mold is the overheated coarse structure and internal crack along the grain boundary,which greatly reduces the strength and toughness of the material.During the use of the mold,it is difficult to withstand the working stress of the die-casting process,and finally the crack is generated at the minimum position of the surface of the mold cavity,that is,the position where the stress concentrated maximumly.